/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
  public boolean hasCycle(ListNode head) {
      if(head==null) return false;
      ListNode fast = head;
      ListNode slow = head;
      while(fast!=null&&fast.next!=null){
          slow = slow.next;
          fast = fast.next.next;
          if(fast==slow) return true;
      }
      return false;
  }
}